"Sometimes a sideways view is most penetrating."
- Edward Tenner
Read Tenner's article in the Atlantic, Write What You Don't Know: http://tinyurl.com/27pvvts
Write about things you have no idea about. We don't always have to be an authority on what we are writing. We can discover knowledge in the process of writing. I think Socrates had it right: that we know everything already, that it is common sense, it's all there - we just need to be hinted at it.
Say that I have a character that I am writing with that is barely known to me. I have a vague idea of how the character's personality operates, but not really how it fits in the big picture or the ins and outs of their behavior. While I write the story, the character will naturally evolve and present itself. As writers, it is sometimes helpful to be surprised by our own writing. This way, our writing is never tiresome or dull, the story we are writing is continually fascinating.
If you are a writer that loves to have everything researched and plotted before you put pen to paper, then choosing a topic you are unfamiliar with is a great way to learn about new information and ways of writing. I recently wanted to write a novel that would be based on pre-independent Afghanistan. I barely know anything about ancient Afghan history, but it intrigued me and caught my interest. If I carried on with my desire to write that novel (I am now focusing on a YA novel instead), I would have research a lot - but I would expose myself to another perspective. If we write only what we know, we are severely limiting our potential to write varied and compelling stories.